1. Field of the Invention
This invention relates to centrifugal pumps of the type known as chopper pumps, and specifically relates to an improved means of axially adjusting the clearances between the pump elements as the elements wear.
2. Description of Related Art
Centrifugal pumps of the type known as chopper pumps are typically employed in processing fluids that contain solid waste materials, such as metal, plastics, municipal waste, animal byproducts, etc. Chopper pumps are structured with cutting elements that operate to chop or cut entrained solids in the fluid into a size that can be disposed of or processed further as needed.
Chopper pumps are typically characterized as having an impeller that is structured with vanes having cutting edges. The cutting edges of the vanes contact a cutting element positioned adjacent the vanes of the impeller to exert a cutting or chopping action on the solid material as it enters the pump. A very close cutting clearance is maintained between the vanes of the impeller and the cutting element to assure proper cutting action on the solids. However, with extended use of the pump, the solids begin to wear down the cutting element and/or the vanes of the impeller such that a gap begins to form between the cutting element and vanes of the impeller. It is critical to the efficiency of the pump to assure that the clearance between the structures is maintained at an appropriate distance or tolerance to optimize cutting action on the solids.
Some chopper pumps are also structured with impellers that have expeller or pump out vanes positioned on the back or drive side of the impeller to assure that solid or stringy material does not get caught between the back side of the impeller and the pump casing, or back plate that may be positioned between the pump casing and the impeller. Additional cutting elements may also be provided on the back side of the impeller to help cut the solids into smaller sizes so that they can be expelled from behind the impeller and not interfere with rotation of the drive shaft. Again, with extended operation of the pump, solids that may infiltrate behind the impeller eventually wear down the cutting elements and/or the expeller vanes, and the clearance between the cutting elements increases with wear. The clearance must then be adjusted to close the gap in order to maintain optimum cutting efficiency behind the impeller.
Conventionally known chopper pumps provide various means for adjusting the pump elements to provide closer cutting clearances. For example, some chopper pumps are adjusted by the insertion of selected sizes of shims between parts of the pump, thereby moving one element closer to another. In other known pumps, adjustment screws are employed. The known adjustment means employed in conventional chopper pumps, however, require the volute casing of the pump to be moved in relationship to the drive casing or bearing frame, or that the volute casing be moved relative to the suction casing, or both the suction casing and drive casing.
Conventionally known adjustment means result in a required change in the mounting dimensions of the pump feet or modification of the piping connection dimensions of the pump, or both. Consequently, the pump must be loosened from its base and some re-alignment performed, either in the drive or piping connections. In lieu of making such re-alignments, the connections must absorb the resulting movement within tolerated levels. Most importantly, such adjustments require that the pump be shut down to effect the required modifications, which translates into increased operation costs.
In addition to the above-noted difficulties that are inherent with conventional adjustment means or devices, movement of the impeller and drive shaft in conventional pumps is ultimately limited because of the sealing mechanisms of the drive shaft. That is, known pump designs have fixed seals about, or associated in some manner with, the drive shaft. Consequently, as the drive shaft is axially adjusted by known methods, the seal working height is also adjusted to the ultimate detriment of the seal and the life of the sealing mechanism is compromised.
Thus, it would be advantageous in the art to provide a centrifugal chopper pump having adjustment means for modifying the cutting clearances between the pump elements such that the adjustment can be effected without having to modify the pump connections or dimensions, without compromising the sealing mechanism and without having to shut down the operation of the pump.